Show variants in ScratchpadAutosuggest parameter tooltip

When a user types `fill();` baby-hints says "fill() takes 1, 3 or 4 parameters, not 0!" but when they type `fill(x)` the tooltip shows `fill(r,g,b,a*)`. It would be nice we could show users either a list of function signatures or maybe one at a time with little arrows to switch. It would be especially nice for the variant that takes a single parameter that we could show `fill(color)` so that users know that they have use a different type.

Start at the ScratchpadAutosuggest parameter tooltip and trace how baby-hints supplies the current function signature. Define how multiple valid fill signatures should be presented, including the single-parameter fill(color) form; done means the tooltip exposes the relevant variants when users type fill() or fill(x).

Written by the indexing model from the issue text.